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KNEE KINEMATICS
The description of the relative motion between
rigid bodies is called kinematics. As the knee
joint is the largest joint of the human body
with an intricate anatomy, its kinematics have
intrigued researchers since a long time [1].
Apart from direct visual observation, the most
popular tool for studying the joint was radio-
graphy. The knee was treated as if it were a
‘planar mechanism’ [2]. In other words, the
movement of the knee was reduced to a two-
dimensional projection of a three-dimensional
reality. In the years following, the limitations
of this methodology became clear, with the
major flaw being the inability to ascertain the
location of the axes of rotation before perfor-
ming kinematic analyses [3]. In 1983, Grood
and Suntay presented a joint coordinate system
providing a geometric description of the three-
dimensional rotational and translational
motion between two rigid bodies, applied to
the knee joint. With this model, the described
joint displacements became independent of the
order in which the component rotations and
translations occur [4]. The new mathematical
insights led to the concept of the helical axis
and opened the door for a correct scientific
description of the kinematics of the knee, allo-
wing six degrees of freedom [5]. However, as
the mathematical accuracy improved, the com-
plexity increased and the model appeared to be
impractical and difficult to apply to the clinical
setting: the clinicians failed to understand the
engineers.
Hollister, and later Churchill, tried to bridge the
gap, reducing the descriptive model to essen-
tially two degrees of freedom [6, 7]. Hollister’s
model described knee motion as pure rotations
occurring around two axes: the so called
‘flexion-extension axis’ and the so called ‘lon-
gitudinal rotation axis’, with the understanding
of the flexion-extension axis not being exactly
located in the coronal plane and the longitudi-
nal axis not being exactly located in the sagittal
plane [6]. As a consequence, these mathemati-
cal ‘simple rotations’ meant in reality flexion-
extension, varus-valgus and internal-external
rotation of the knee joint, confusing the clini-
cian trying to apply this knowledge to the prac-
tical setting. Churchill addressed this problem
by allowing a mathematical error in the kine-
matic description, based on a loaded rig expe-
riment with an ankle load of 100 N and a com-
bined hamstrings load of 30N [7]. The advan-
tage of this approach was the link between the
kinematic description and certain anatomic
landmarks, allowing clinicians to apply this
knowledge in practice.
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COMPARATIVE KINEMATICS BETWEEN
THE NATIVE KNEE AND TOTAL KNEE
ARTHROPLASTY
J. VICTOR